The present invention relates to a method for preventing polymer scale deposition on the reactor walls in the polymerization of an ethylenically unsaturated monomer such as vinyl chloride and styrene and a coating composition used therefor. The invention also relates to a method for preventing adhesion of polymers on the walls of a storage tank or salting-out tank of polymer latexes as well as walls of apparatuses used for processing of polymer products.
Ethylenically unsaturated monomers are polymerized in several types of polymerization procedures including suspension polymerization, emulsion polymerization, solution polymerization, gas-phase polymerization and bulk polymerization. One of the difficult problems common to all of these polymerization procedures in the prior art is the deposition of polymer scale on the inner walls of the reactor and the surfaces of other equipments such as the stirrer and the like coming into contact with the monomer under polymerization.
That is, when an ethylenically unsaturated monomer is polymerized in either one of the above mentioned polymerization procedures, the polymer is deposited more or less on the inner walls of the reactor and the surfaces of the stirrer and other equipments coming into contact with the monomer to form scale thereon so that various disadvantages are unavoidable such as the decrease in the yield of the desired polymer and lowered cooling capacity of the polymerization reactor as well as degraded quality of the polymer product due to the polymer scale eventually entering the polymer product as coming off the reactor walls. In addition, such polymer scale on the reactor walls must be removed taking a great deal of time and labor in order to prepare the polymerization reactor for the next run of the polymerization. The scale removing works by man power present a very serious problem of safety against human body because the polymer scale usually contains a considerably large amount of the unreacted monomer absorbed therein which is sometimes toxic as is a matter of very serious concern in recent years in the case of vinyl chloride in particular.
A number of methods have been proposed hitherto to prevent the polymer scale deposition on the reactor walls. Exemplary of such methods, in particular, for the suspension polymerization of vinyl chloride are a method in which the reactor walls and the surfaces of the stirrer and the like are coated, prior to polymerization, with a polar organic compound such as amine compounds, quinone compounds, aldehyde compounds and the like or a dye or pigment (see, for example, Japanese Patent Publication Nos. 45-30343 and 45-30835), a method in which the walls and surfaces are coated with a polar organic compound or a dye having been treated with a metal salt (see Japanese Patent Publication No. 52-24953), a method in which the coating material is a mixture of an electron donor compound and an electron acceptor compound (see Japanese Patent Publication No. 53-28347) and a method in which the coating material is an inorganic salt or an inorganic complex compound (see Japanese Patent Publication No. 52-24070).
Each of these prior art methods, however, has its own respective advantages and disadvantages. For example, difficulties are encountered in the coating works with some of the coating compositions and the velocity of the polymerization reaction may sometimes be retarded by the influence of the scale-preventing coating composition or the quality of the product resin obtained by use of a large amount of the coating composition may be poor due to the contamination of the resin product with the ingredients in the coating composition. Some of the ingredients in the hitherto proposed coating compositions are even corrosive to the material of the polymerization reactor. Accordingly, it has been eagerly desirted to develop an efficient method for preventing deposition of polymer scale on the walls of a polymerization reactor as well as other apparatuses used for processing the polymer products.